The transmitter power control specifications for code division multiple access (CDMA) data and telephony systems, such as 3GPP, WCDMA and CDMA 2000 impose tough requirements for the operation of these “3G” handsets. In general a base station communicating with a plurality of handsets within a cell needs all the signals arriving at the base station to have substantially the same power level. If this is not the case, then those signals with greater intensity become on-channel interferers for those signals of reduced intensity and eventually could cause the cell to cease operation.
In general the gain of the transmitter in a mobile device, such as a handset and data card, varies with transmit frequency, ambient and die temperature, supply voltage and fabrication process tolerances. However the “3G” standards require that the mobile unit should be able to change its output power in predetermined step sizes in response to instructions from the base station. Typically the mobile handsets are expected to be able to make 1 dB changes in output power with a 0.5 dB accuracy, and 10 dB step changes in power with a 2 dB accuracy. Furthermore the absolute maximum power transmitted by the base station should be controllable to within 2 dB.
In order to achieve the specified step sizes over the required dynamic range, manufacturers have generally used one or other (or a combination) of two techniques. In a first approach the handset is extensively calibrated during production testing in order to characterize its response over at least frequency, and optionally battery voltage. Calibration over temperature is generally avoided as this is time consuming. A second approach is to use a power detector during the normal transmit operation. The power detector is generally in the form of a logarithmic amplifier or received signal strength indicator, which generally takes hundreds of microseconds to take a measurement. This prevents the detector from being used on a slot-by-slot basis within the handset in order to control the transmit power within a slot and therefore the calibration data is combined with occasional use of a detector at high transmit power levels to prevent the handset from exceeding the maximum transmit power allowed by the relevant 3G standard. It should be noted that the CDMA signal looks like noise, so generally a relatively long integration time is required for these detectors in order to achieve a reliable power measurement. Each WCDMA slot lasts around 670 microseconds with around 1500 power correction commands being received by a mobile telephone from the base station every second.